Preparation,Properties And Theoretical Calculation Of Ferric Fluoride Cathode Materials For Lithium-Ion Batteries

This dissertation mainly focuses on the experimental synthesis,electrochemical performance research and theoretical calculation of ferric fluoride cathode materials.,The specific research includes the following sections:Firstly,the C/FeOF/FeF3 nanocomposites were synthesized by a simple in-situ partial oxidation method.Moreover the high resolution transmission electron microscope?HR-TEM?was used to show that the C/FeOF/FeF₃ nanocomposites consist of FeOF and FeF₃ interpenetrating nano-domains.The lithium ion diffusion coefficient was measured by cyclic voltammetry?CV?and electrochemical impedance spectroscopy?EIS?.The results show that the introduction of nanoscale FeOF enhances the electronic conductivity and ionic conductivity of the composites.At a voltage range of 1.5-4.5V and a current density of 20 mA·g^-1,the first ten cycles specific capacitance of the C/FeOF/FeF₃ nanocomposite measured by the constant current charge-discharge test was maintained at around 400 mAh·g^-1.and it made the material have a good cycle capacity retention rate.At current densities of 100 mA·g^-1 and 200 mA·g^-1,stable capacities of 80 mAh·g^-11 and 60 mAh·g^-11 were observed,respectively.Furthermore,the capacity in the subsequent cycle was almost constant and the discharge platform based on embedding and conversion reactions also increased by about 0.4V.These phenomena enhanced the outstanding performance of high voltage capacity.Secondly,the diffusion kinetics of Li⁺intercalation/deintercalation in C/FeOF/FeF₃nanocomposites synthesized by the in-situ partial oxidation method was studied.In order to obtain the lithium diffusion coefficient of the material,three different test methods were used in this paper:electrochemical impedance spectroscopy?EIS?,cyclic voltammetry?CV?and constant current intermittent titration?GITT?.In the 1.55-4.5V voltage range,the EIS method showed that D_Li+varied in the range of 9.0×10^-15-2.87×10^-14 cm²·s^-1.During the charging/discharging process of C/FeOF/FeF₃ nanocomposites,the calculation of D_Li+using the GITT method was in the range of 7.0×10^-12-6.18×10^-11cm²s^-1.The results of cyclic voltammetry for different scanning rates(0.1,0.2,0.3,0.5,1.0and2.0mVs^-1)indicated that the D_Li+of C/FeOF/FeF₃ nanocomposites during the anion/anode redox process were 1.012×10^-12cm²s^-1,1.516×10^-12cm²s^-1.According to the results of D_Li+measurements for the C/FeOF/FeF₃ electrodes obtained from three different test methods,the C/FeOF/FeF₃ nanocomposites can be used as high-performance cathode materials for lithium-ion batteries.In the end,the crystal structure and electronic structure properties of FeF₃ and FeOF/FeF₃materials under antiferromagnetic conditions were mainly studied using the GGA+U method based on density functional theory:The FeF3 and FeOF/FeF3 crystal models were constructed using Materials Studio platform,The geometry,energy band structure,and density of states of the two materials were analyzed.The results of the study show that the crystal structure parameters after optimization are almost consistent with the experimental data.FeF₃ was an insulating material with a large energy gap?approximately 4.1eV?,and FeOF/FeF3 with an O-substituted Fe moiety had a smaller band gap of 1.68eV,which fundamentally improved the electrical conductivity of the FeF₃ material.